Observation of Postsunset OI 135.6 nm Radiance Enhancement Over South America by the GOLD Mission

The Global-scale Observation of Limb and Disk (GOLD) mission, for the first time, provides synoptic two-dimensional (2D) maps of OI 135.6 nm observations. These maps describe the unambiguous and dynamic evolution of nighttime ionospheric F₂-peak electron densities (N_mF₂) as the 135.6 nm airglow emission radiance correlates well with N_mF₂ at night. On November 19, 2018, the 135.6 nm radiance measured by GOLD, N_mF₂ measured by a digisonde, and GPS total electron content (TEC) measurements at Cachoeira Paulista (CP) all showed a postsunset enhancement, with an increase near 22:30 universal time. The 135.6 nm radiance map showed that this enhancement was due to the southward movement of the southern equatorial ionization anomaly (EIA) crest. Therefore, the GOLD observation showed the linkage between postsunset enhancement of N_mF₂ and EIA movement. Furthermore, unlike the southward movement of the southern crest, the corresponding EIA northern crest, however, did not show northward motion. This is the first time that the EIA hemispheric asymmetry, which included both different densities and movement of two crests in a short time period (<2-h), was captured. The cause of this asymmetric movement of the two crests is not clear and requires further investigation.